Let's face it, you really don't have to be a tech wizard to know that Google is the leader in online search. It's no surprise that the puny 3% share that Microsoft's Bing has is not enough to make Google sweat and Microsoft happy. In an effort to increase Bing's market share, Microsoft has made a change tied to the installation of its Outlook app on Android.
According to a post on the Pixel Phone Help page, a long press on Outlook will now bring up a trio of options for the user including one that includes a Bing search on the text being highlighted. If that option is tapped, a browser opens inside Outlook with Bing loaded up and the results of the search already done. So in other words, even if you don't have the Bing app installed on your phone, a Bing search is what you're going to get.
Microsoft is employing a feature that Google added with Android Marshmallow that allows developers to add their own options to the menu that surfaces when words are selected. Even though this is a legitimate Android feature that Microsoft is using to its advantage here, asking a user to run a Bing search on a device without the Bing app installed might be considered as unseemly as begging for change at a red light.
If you want to eliminate this feature, all you need to do is uninstall Outlook on your Android handset and the option to have a Bing search performed will be gone from your phone.